- Drop loaded_python3_provider + python3_host_prog: the guard variable
was wrong (1 doesn't disable, 0 does) and the venv path doesn't
survive a fresh clone. Let nvim find system python instead.
- Add "black" to Mason ensure_installed; conform already uses it for
Python but it wasn't being auto-installed.
- Delete lua/custom/configs/rust-tools.lua: leftover from before
rustaceanvim migration, never required anywhere.